1. Field of the Invention
The present invention relates to a video projector, and more particularly to a video projector for time-dividing a light from a white-light source to light fluxes having given colors to be displayed.
2. Description of the Prior Art
As a video projector for displaying a colored image, there has been a video projector which time-divides a light from a white-light source into three primary-colored light fluxes, thereafter modulates intensities of the three primary-colored light fluxes subjected to time division and combines them for display.
Japanese patent laid-open publication No. Sho 63-316590 (disclosed on Dec. 23, 1988 in Japan) discloses a color selection scanning device for time-dividing a light from a white-light source into three primary-colored light fluxes. There is disclosed a video projector using a color wheel in which three colored translucent filters for transmitting each colored light flux, i.e., a red-colored light flux, a green-colored light flux and a blue-colored light flux therethrough are radialized in a circle, and the technique shown in this publication will be described as a prior art.
FIG. 16 is a perspective view showing the structure of a conventional video projector.
A light from an optical source 101 enters a filter plate 102 so that a red-colored light flux, a green-colored light flux and a blue-colored light fluxesequentially penetrate in synchronism with a rotation of the filter plate 102. Each colored light flux that has penetrated the filter plate 102 enters a flat display 103.
The flat display 103 displays images associated with a red-colored light video signal, a green-colored light video signal and a blue-colored light video signal in synchronism with switching between the red color, the green color and the blue color of the filter plate 102 and modulates intensities of the incident light fluxes for each pixel. The penetrated light fluxes from the flat display 103 are magnified and projected on a screen 105 through a lens system 104.
In the above structure, a velocity and a phase of rotation of the filter plate 102 are controlled in such a manner that colors of the incident light fluxes sequentially changes from the upper side of the filter plate 102 downwards in synchronism with that the image on the flat display 103 are sequentially scanned from the lower side every one line upwards in the horizontal direction.
As described above, in the conventional video projector, a color-projected image is obtained by using the filter plate in which the three elementary colors are radialized as the color selection scanning device.
However, the prior art has the following problems.
Firstly, there is such a problem as that the color purity of an image projected on the screen 105 is low. That is because the optical source 101 is not an ideal point optical source that is infinitely small in size and has a finite largeness expanding to a predetermined range. Light fluxes radiated from the optical source 101 thus contain various angle components, and complete parallel light beams can not be obtained. In this conventional apparatus, since the filter plate 102 and the flat display 103 are provided with a predetermined distance therebetween, a color border formed by the filter plate 102 spreads on a surface of the flat display 103 and adjacent colored lights on the filter plate 102 are mixed.
Secondly, there is a problem of generation of color shading in a projected image. This problem occurs because the flat display 103 is sequentially linearly scanned upwards every one line in the horizontal direction whereas the filter plate 102 is scanned so as to rotate around the rotational axis. FIG. 17 is a partially-enlarged front view showing the filter plate 102 in the video projector illustrated in FIG. 16. Although a color border 102B of the filter plate 102 is horizontal on a horizontal line H running through the rotational axis of the filter plate 102, the inclination of the color border 102B of the filter plate 102 with respect to the horizontal line H becomes large as the color border 102B is separated from the horizontal line H, and a color-mixed area R in which a color of the penetrated light from the filter plate 102 does not coincide with a color modulated by the flat display 103, generating mixture with other colors in the color-mixed area R.
Thirdly, an attempt to suppress color shading in the projected image leads to an increase in size of the apparatus. As apparent from FIG. 17, this problem is generated because a size of the color-mixed area R must be decreased in order to reduce the color shading, and hence a diameter of the filter plate 102 must be set large as possible with respect to the size of the flat display 103 in such a manner that the color border of the filter plate 102 does not largely incline toward the horizontal line H.
Fourthly, reduction in color shading of the projected image involves decrease in brightness of the projected image. That is because provision of a light blocking portion 102X in a predetermined angle range in the radial direction around the color border 102B of the filter plate 102 for the purpose of avoidance of penetration of the light in the color-mixed area R causes the light blocked by the light blocking portion 102X to be disused, thereby reducing the efficiency of the light availability.
An object of the present invention is to provide a video projector which can obtain high brightness of a projected image, high color purity and low color shading and which can be downsized and reduce the manufacturing cost.
To realize the above-mentioned object, the first aspect of the present invention is that a video projector having: an optical source, a color selection scanning device for selectively filtering a color of a light from the optical source to generate a light flux having a specific color in synchronism with a supplied video signal; a video displaying device for modulating an intensity of the light flux in synchronism with the video signal pixel by pixel to form a modulated image responsive to the video signal; an imaging lens disposed between the color selection scanning device and the video displaying device, for imaging an intensity distribution and a color distribution of the light flux outgoing from the color selection scanning device on a surface of the video displaying device; and a projecting lens for projecting the modulated image formed by the video displaying device.
According to the second aspect of the present invention is that a video projector having: an optical source; a color selection scanning device for selectively filtering a color of a light from the optical source to generate a specific color in synchronism with a supplied video signal; a color separating device for separating a light flux transmitted through or reflected on the color selection scanning device into first and second light fluxes having different wavelengths; a color combining device for combining the first and second light fluxes separated by the color separating device; a first video displaying device disposed between the color separating device and the color combining device, for modulating an intensity of the first light flux in synchronism with the video signal pixel by pixel to form a first modulated image responsive to the video signal; a second video displaying device disposed between the color separating device and the color combining device, for modulating an intensity of the second light flux in synchronism with the video signal pixel by pixel to form a second modulated image responsive to the video signal; a first imaging lens disposed between the color separating device and the first video displaying device, for imaging an intensity distribution and a color distribution of the light flux outgoing from the color selection scanning device on a surface of the first video displaying device; a second imaging lens disposed between the color separating device and the second video displaying device, for imaging an intensity distribution and a color distribution of the light flux outgoing from the color selection scanning device on a surface of the second video displaying device; and a projecting lens for projecting the first modulated image and the second modulated image formed by the first video displaying device and the second video displaying device.
In the present invention, since the imaging lens, i.e., a relay lens is provided between the color selection scanning device and the video displaying device, each colored light fluxesubjected to time division manner by the color selection scanning device can be precisely imaged on a surface portion of the video displaying device, thereby improving the color purity of the projected image without generating color mixture on the surface of the video displaying device.
Further, the size of a virtual image of the video display device on the surface of the color section scanning device can be arbitrarily set by appropriately selecting each distance between the color selection scanning device, the imaging lens and the video displaying device and a focal distance of the imaging lens and changing a magnification of the imaging lens. For example, when using a color selection scanning device which is easily affected by heat, the radiation efficiency of the color selection scanning device can be improved by setting the magnification of the imaging lens to less than 1 and increasing the size of a virtual image of the video displaying device on the surface of the color selection scanning device, thus suppressing an increase in temperature.
On the contrary, when the magnification of the imaging lens is increased and the size of a virtual image of the video displaying device on the surface of the color selection scanning device is reduced, the color selection scanning device which is small in size can be used, resulting in realization of miniaturization of the apparatus and reduction in the manufacturing cost.
Moreover, color mixture is not generated in the border area even if the light blocking portion is provided in the color border area of the color selection scanning device, and the width of the light blocking portion can be greatly reduced as compared with the prior art, further improving the brightness of the projected image.